1. Field of the Invention
The present invention relates to a medical X-ray computed tomography apparatus.
2. Description of the Related Art
A conventional medical X-ray CT apparatus is, as disclosed in JP-B 3587410, provided with internal devices such as X-ray tube and X-ray detector for taking tomographic images, and these internal devices are mounted on a pedestal and covered with a gantry cover, whereby a gantry of the X-ray CT apparatus is configured. This gantry cover is made of a light-weight resin material and is formed to be thin-walled in order to facilitate maintenance/checkup work. The gantry is provided with a bore into which a bed supporting a patient is inserted, and the patient lying on the bed is irradiated with X-rays to thereby acquire a tomographic image. This bore is surrounded by the gantry cover and formed in such a manner that, the bore is expanded from the central part thereof toward the opening part thereof in order to improve the operative environment at the time of subjecting the patient to tomography, diagnosis, and treatment, and give a sense of space to the patient to alleviate the mental burden on the patient.
In such a medical X-ray CT apparatus, the X-ray tube and X-ray detector are mounted on a rotary portion of the pedestal in such a manner that the X-ray tube and X-ray detector are opposite to each other across the bore. During the time of tomography imaging and rotation of the rotary portion, the patient is irradiated with X-rays emitted from the X-ray tube, and the X-rays transmitted through the patient are detected by the X-ray detector and converted into an X-ray detection signal. This X-ray detection signal is processed by a computer, and a tomographic image associated with the body of the patient is acquired.
During the tomography imaging, wind arises inside the gantry due to the rotation of the rotary portion, and the gantry cover vibrates due to the air pressure of the wind, and vibration induced noise is generated. Particularly, the expanded cover portion of the gantry cover surrounding the bore is arranged close to the rotary portion, and is therefore forced to vibrate strongly by the force of the wind. Further, the expanded cover portion is provided to surround the bore, and hence the vibration induced noise generated at the expanded cover portion is concentrated in the bore. Particularly, when the expanded cover portion is forced to vibrate at a frequency corresponding to the natural frequency thereof, the vibration of the expanded cover portion is increased, and the vibration induced noise is also increased.
In recent years, in the X-ray CT apparatus, a trend toward high-speed scanning has advanced, and the rotary portion tends to be rotated at higher speed. Consequently, the air pressure produced inside the gantry is increased, and the vibration of the expanded cover portion is also increased. As a result of this, the vibration induced noise is increased, and is perceived as intrusive. Thus, the vibration induced noise will disturb the medical practice and make the patient feel uneasy.
As a solution for reducing the vibration of the gantry cover described above, increasing the flexural rigidity of the gantry cover by adding a reinforcing material such as glass fiber into the resin material for forming the gantry cover is conceivable. However, when the constituting ratio of the reinforcing material becomes large, the formation and the manufacturing of the cover become difficult. Further, as another solution, making the flexural rigidity larger by uniformly increasing the thickness of the gantry cover is conceivable. However, this leads to an increase in weight of the gantry cover, and the labor required for transportation and assembly of the gantry cover is increased, and the efficiency at the time of maintenance/checkup is lowered.
In an X-ray CT apparatus disclosed in JP-A 09-140697 (KOKAI), a touch sensor is fitted into a fitting hole formed by substantially linear beam members on an expanded cover portion, and is fixed by mechanical connection means such as screws and the like. Although the vibration of the expanded cover portion can be reduced by fixing the touch sensor to the expanded cover portion with the mechanical connection means, a large number of fixing places must be provided, and generally it is difficult to secure sufficient fixing places. Further, providing a large number of fixing places leads to an increase in labor at the time of assembly, and lowering of the efficiency at the time of maintenance/checkup, and hence this method is impractical. Although it is also possible to reduce the vibration by increasing the cross section size of the substantially linear beam members, however, in order to reduce the vibration of expanded cover portions with large openings it is necessary to make the cross section size large, and this is difficult because the size of cross section is limited by the restriction on the outer dimensions. Further, increasing the cross section size of the beam members leads to an increase in weight of the expanded cover portion.
As described above, in the conventional medical X-ray CT apparatus, the gantry cover vibrates due to a wind generated inside the gantry by the rotation of the rotary portion, and vibration induced noise is generated, and the vibration induced noise makes the patient feel uneasy. From this point of view, a medical X-ray CT apparatus in which the vibration of the gantry cover and the intrusive vibration induced noise can be reduced without greatly increasing the weight of the gantry cover is demanded.